1. Field of the Invention
This invention relates to combustors for gas turbine engines, including the associated fuel burners, otherwise known as fuel nozzles or injectors. It particularly, but not exclusively, relates to combustors suitable for operating in a so-called xe2x80x9clow emissions combustion modexe2x80x9d because they can sustain a premixed lean-burn combustion processes.
2. Description of the Related Art
Some known types of combustor utilize a premixed lean-burn combustion mode to reduce emissions of pollutantsxe2x80x94such as nitrous oxides (xe2x80x9cNOxxe2x80x9d), carbon monoxide (xe2x80x9cCOxe2x80x9d) and unburned hydrocarbons (xe2x80x9cUHCxe2x80x9d)xe2x80x94by enabling combustion temperatures to be kept within certain limits known to minimize pollutant production. To achieve such a low emissions combustion mode, air is mixed with the fuel before initial injection of the fuel/air mixture into a combustion zone in the combustion chamber. The term xe2x80x9clean-burnxe2x80x9d implies that the amount of air mixed with the fuel is more than that required for complete combustion of the fuel.
Gas turbine engines operate over a wide range of engine speed and load conditions, varying from initial start-up conditions, through various engine speed/load combinations up to a maximum. When an engine""s combustor is operating in a premix lean-burn low emissions combustion mode, it is well known that flame stability is difficult to achieve in the lower power ranges, because the combustion process is being operated close the weak extinction limit of the air/fuel mixture. An associated problem is non-uniformity of the combustion process, leading to undesirable variation in the temperature and composition of the combustion products leaving the primary combustion zone and increased temperature gradients in the combustor walls nearest the combustion process. Such non-uniform combustion is caused by insufficiently rapid homogenization of the fuel/air mixture before it ignites in the flame front established in the combustion chamber.
A well known way of stimulating good mixing of air and fuel is to impart a swirling motion to the fuel and air, either before, during or after the injection of fuel into an airstream. Published European patent specification EP 0 378 505 B1 discloses how, in an annular combustion chamber, fuel injectors having associated air swirlers can be used to project fuel and air along or parallel to the axial centerline of the combustion chamber. The fuel injectors are arranged in radially inner and outer circumferentially extending tiers or rows in the combustor head and the fuel is mixed with the swirling air after the fuel leaves the ends of the injectors within the combustion chamber. The air swirlers associated with the injectors in one row swirl the air in one rotational direction, whereas the swirl imparted to the air by the air swirlers in the other row is in the opposing rotational direction. Additionally, the fuel injectors in each row are in angular registration with the gaps between injectors in the other row so that the fuel and air discharged from injectors in one row interacts with the discharge from two adjacent fuel injectors in the other row. This is said to establish a reinforced fuel/air swirl pattern in the front or head of the combustor for stabilizing the burning. However, the object of this prior invention is to increase the intensity of combustion in non-premixed diffusion controlled combustion processes, whereas for premixed lean-burn combustion processes aimed at reduced emissions, the combustion intensity has to be controlled within certain limits.
Stable premixed lean burn combustion can be extended into somewhat lower power ranges by the expedient of performing the combustion process in a number of stages, for example as shown in International patent publication number WO92/07221. This discloses a sequentially staged combustion process comprising primary, secondary and if necessary tertiary stages of combustion occurring in sequentially arranged combustion chambers, with the chamber containing the primary combustion zone feeding its combustion products into the secondary stage and so on. A disadvantage of such sequentially staged combustion processes is the extra length required of the combustor in order to ensure an adequately complete combustion process in each stage before further premixed fuel and air is added to the combustion products to initiate the next stage of combustion.
European patent publication EP 0728 989 A2, to the present assignee, discloses a lean burn combustor in which the main premixing fuel burner or injector comprises an annulus surrounding a central pilot fuel burner or injector. The main premixed fuel-lean fuel/air mixture is injected into the combustion chamber as an initial radially inwardly moving swirling flow which meets an axially directed air-blast from the pilot burner. This enhances homogenization of the main fuel-lean fuel/air mixture when it enters the combustion chamber, and aids in avoiding its premature ignition. The pilot air-blast is configured to provide a region of sheltered combustion which supports the combustion process just downstream of the pilot burner and increases stability of the premixed lean burn combustion process at part power engine conditions. This arrangement facilitates a so-called xe2x80x9cparallel stagedxe2x80x9d or xe2x80x9cfuel stagedxe2x80x9d combustion process, in which both primary and secondary combustion processes take place at the same streamwise axial position, fuel being appropriately apportioned between the pilot and main burners according to a schedule of fuel flows against power level. Both a low emissions combustion mode over a wide power range and a relatively short combustion chamber are achieved. Nevertheless, the necessary proximity of the premixed lean-burn main flame and the fuel-rich pilot flame to the pilot burner tends to cause high temperatures in the exposed end of the pilot burner, and the fuel-rich pilot flame also causes the formation of some NOx at the low engine power levels for which it is used.
Of course, practical designs are inevitably a result of compromises between conflicting requirements, but it will be understood from the above that designs of relatively short combustors are required which facilitate not only stable low emissions combustion over a wide power range but also a uniform combustion process in a primary combustion zone which has adequate separation from the combustor head.
According to the present invention, a combustor for a gas turbine engine comprises
an annular combustion chamber having radially inner and outer concentric walls and a combustor head wall at its upstream end, and
a plurality of fuel injectors disposed in the combustor head wall,
the fuel injectors being circumferentially spaced around the combustor head wall and disposed as a first radially outer row of injectors and a second radially inner row of injectors, the rows being concentric with the combustor head wall, the injectors in the first and second rows being disposed such that injectors in each one of the rows are in angular registration with spaces between injectors in the other row;
wherein each fuel injector comprises
means for producing a fuel/air mixture having a swirling motion, and
a mixing duct located downstream of the means for producing the fuel/air mixture, the mixing duct opening into the combustion chamber through the combustor head wall and having a length sufficient to allow at least partial homogenization of the fuel/air mixture before entry to the combustion chamber as a divergent swirling stream,
the mixing ducts in the first and second rows of injectors having longitudinal axes oriented to coincide with generating rays of respective first and second imaginary conical surfaces, which conical surfaces intersect within a primary combustion zone in the combustion chamber, whereby mixing ducts located in different rows are angled towards each other in the downstream direction and the divergent swirling streams of fuel/air mixture coming from different rows cross in the combustion chamber in an interdigitating manner, thereby creating a strong mixing interaction between the streams from different rows and enhancing uniformity of combustion in the primary combustion zone.
As used above, the term xe2x80x9cconical surfacesxe2x80x9d includes the case where one, and only one, of the conical surfaces is in fact a cylindrical surface, a cylinder being defined as a cone whose apex is infinitely distant.
It is believed that the strong mixing interaction of the crossed interdigitated swirling streams is due to turbulent interference between the swirling motions of the streams"" outer layers as they interact with each other. As it moves downstream, the turbulence also rapidly propagates transversely of the downstream direction to produce a substantially homogeneous fuel/air mixture with a desirable uniformity of the combustion products leaving the primary combustion zone. The turbulent interaction between the crossing streams of fuel/air mixture is presently believed most effective if the direction of swirl motion is opposite for adjacent injectors in each row. However, the invention also includes alternative second and third arrangements in which the direction of swirl motion is the same for streams coming from all the injectors, or in which it is the same for injectors in the same row as each other, but opposite for injectors in different rows. Currently, we believe that the third arrangement does not achieve such efficient mixing as the first and second, but may well facilitate better light-round of the annular combustor during the engine ignition sequence. For stationary land-based gas turbine engines, in which low emissions performance may be given precedence over good ignition characteristics, the first and second arrangements may be more advantageous, whereas for vehicular gas turbines, particularly aeroengines, the hoped for good ignition characteristics of the latter arrangement may be preferred.
Combustors designed in accordance with the invention, having a more uniform combustion process and temperature in the primary zone and downstream thereof, will therefore experience less severe temperature gradients in the combustor walls, particularly in the combustor head wall. This can facilitate a longer combustor wall life due to lower stresses in the wall materials. Furthermore, the way in which mixing ducts located in different rows are angled towards each other, as stated above, combines synergistically with the enhanced mixing action to reduce the length of the primary combustion zone, enabling use of a relatively short combustion chamber, thereby economizing on space, weight, cost of materials and amount of cooling air required for cooling the combustor wall.
The included angle between the first and second imaginary intersecting conical surfaces, hereinafter termed xe2x80x9cthe angle of intersectionxe2x80x9d may be chosen as a compromise between a wide angle and a narrow angle. A wide angle could lead to poor separation of the primary combustion zone from the combustor head wall and excessive impingement of the combustion process on the radially inner and/or outer walls of the combustion chamber. On the other hand, a narrow angle would substantially avoid such impingement but extend the primary combustion zone by weakening the mixing interaction between the streams, thus reducing the beneficial effects of combustion temperature evenness and also requiring a longer combustion chamber. In one example of the invention, the angle of intersection is between 30xc2x0 and 50xc2x0 (preferably 40xc2x0 ). However, the angle may be more or less than this range, depending on the combustion chamber""s configuration and its desired dimensions.
A combustor designed in accordance with the invention can be operated in a parallel- or fuel-staged low emissions combustion mode over a wide predetermined power range. This can be achieved in the following way:
During operation over a high power part of the wide predetermined power range, the fuel/air ratio of each individual injector can be such that the fuel-air mixture it emits can support stable lean-burn combustion. In this case, the low emissions combustion mode can be sustained by supplying equal amounts of fuel to all the injectors.
During operation over the lower part of the wide predetermined power range, (i.e., while the engine power is being xe2x80x9cturned upxe2x80x9d to, or xe2x80x9cturned downxe2x80x9d from, high power operation) a plurality of injectors in at least one of the rows (the injectors of said plurality being equally spaced around the at least one row and interspersed with the other injectors in the row) are supplied with reduced quantities of fuel as compared with the other injectors. These reduced fuel injectors can have their individual fuel/air ratios reduced below the level where stable combustion can be maintained, while the other injectors are fueled at a level which supports stable combustion. By this means, the injectors with the greater fueling sustain the whole combustion process within the combustor, even though the fuel/air ratio averaged over all the injectors may be less than that required for stable combustion.
The divergent swirling streams issuing from the reduced fuel injectors act to dilute the fuel/air mixture which enters the primary combustion zone from the more highly-fueled injectors. In prior art parallel staged annular combustors, it is frequently found that such dilution quenches the combustion process during low power operation, leading to excessive production of CO and UHC, and unstable combustion. Consequently, the lean-burn combustion process has had to be used over a restricted power range. However, the present combustor is particularly advantageous when used in a premixed lean-burn parallel staged combustion process because the rapid mixing obtained in the primary combustion zone enables premixed lean-burn operation to be extended to lower powers than hitherto without development of unstable combustion and undesirable combustion products.
Conveniently, it may also be arranged that during operation of such a parallel staged combustor over a predetermined low-power range, in which an overall lean-burn combustion process would be too unstable, some of the fuel injectors are fueled such that they locally produce a fuel/air mixture which is richer than that used for the low emissions combustion mode. By this means, local hot flame pilot combustion processes occurs in the primary combustion zone to stabilize the overall combustion process during the transition between zero power and the low emissions combustion mode.
Although a combustor according to the present invention is particularly advantageous for enabling a gas turbine engine to operate with a predominantly premixed lean-burn combustion processes, the same principle of combustor construction can of course also be used for engines operating only with a diffusion flame combustion process. In this case, the invention can be used to reduce UHC""s at low power, carbon production and temperature non-uniformity.